Task: Applicative functions with context

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Information

Sometimes you want to use map, filter, reduce or other functions inside an object and you want want to access something on this. Thankfully this is very simple on _.filter and friends – just add the context, i.e. what you want this to be, as the last parameter.

describe('filter with context', function() {
    var ages = [20, 30, 75, 156, 200];

var people = {
      maxAge: 120,
      validAges: function(ages) {

return _.filter(ages, function(age) {
          return age < this.maxAge;
        }, this);

// here we specified that we want `this` inside the function in the
        // filter be the same as `this` is inside the current function

}
    };

it('works by passing a context to the lodash-function which it can bind to', function() {
      var result = people.validAges(ages);

expect(result).to.deep.equal([20, 30, 75]);
    });
});

describe('our filter with context', function() {

// PROBLEM: Fix our filter implementation below so it handles the
  // specified context

var filter = function(items, fn, context) {
    var arr = [];
    for (var i = 0; i < items.length; i++) {
      if (fn(items[i], i)) {
        arr.push(items[i]);
      }
    }
    return arr;
  }

var ages = [20, 30, 75, 156, 200];

var people = {
    maxAge: 120,
    validAges: function(ages) {
      return filter(ages, function(age) {
        return age < this.maxAge;
      }, this);
    }
  };

it('works when we pass a context that we bind the function to', function() {
    var result = people.validAges(ages);
    expect(result).to.deep.equal([20, 30, 75]);
  });

});

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Available globals

_: Lo-dash.js
log: log-command that logs to view and console

Testing

describe: new test suite (mocha)
it: new test (mocha)
xit: pending test (mocha)
xdescribe: pending scenario (mocha)
before/after: run code before or after all tests (mocha)
beforeEach/afterEach: run code before or after each tests (mocha)
chai: chai.js assertion library